Article and method of heat treating high phosphorus steels



United States Patent ICE 3,438,821

Patented Apr. 15, 1969 3,438,821 sitic hardening techniques in which the tempering step ARTICLE AND METHOD OF HEAT TREATING lIlVOlVeS cooling at a rapid rate from 3. narrow range Of HIGH PHOSPHORUS STEELS temperatures just below the Ac It has been discovered John Y. Ri d l, B thl h P assignor t B thl h that accelerated cooling from the tempering temperature Steel Corporation, a corporation of Delaware 5 has a very pronounced and unexpected effect on the im- No Drawing. Continuation of ap lication Ser. No. pact transition temperature of martensitically hardened 497,550, 1965- Thls appllcatloll y 17, medium carbon-high phosphorous steels which does not 1963, 2 3 C 2 M 1/22 occur in medium carbon-low phosphorous steels heat Us. CL n 7 Claims treated in a similar manner. This discovery has made it 10 possible to extend the use of phosphorous as an alloying element into the field of martensitically hardened steels which have been hitherto considered to be too brittle.

ABSTRACT OF THE DISCLOSURE Broadly, the tempering treatment of this invention comprises heating martensitically hardened, high phosphorous steel to a temperature between the AC1 temperature and about 150 F. below the Ac and cooling from the tempering temperature at a rate of not less than 1 F. per

second. With alloy steels having about 30% to .45%

carbon, the tempering temperature should preferably be between 1250 F. and 1300 F. and oil quenching from the temper provides a satisfactory cooling rate for sections up to 2" in diameter.

The tempering treatment of my invention is applicable to steels containing not less than .12% carbon and not less than .06% phosphorous. It is particularly useful in the treatment of high strength, low alloy corrosion resistant steels containing from 35% to .55% carbon, .06% to .12% phosphorous, and other elements such as copper, nickel and/or chromium added to enhance the corrosion resistance of the steels. Compositions illustrating the broad and narrow limits of these steels are as A method of tempering martensitically hardened high phosphorus-carbon steel to provide a combination of corrosion resistance, high strength and low impact transition temperature. The tempering treatment comprises heating the martensitically hardened steel to a temperature between the Ac temperature and about 150 F. below the Ac and cooling from such temperature at a rate not less than 1 F. per second.

This application is a continuation of application Ser. No. 497,550, filed Oct. 18, 1965, now abandoned.

This invention relates to a method of heat treating steel and more particularly to a method of improving the notch toughness of martensitically hardened high phosphorous steels.

In the past, the use of significant amounts of phosphorous as an alloying element has been limited to low carbon ferritic steel. The principal reason for this limitafollows" tion appears to be the embrittling efifect of phosphorous. Narrow Broad It is well known that this eifect is particularly severe in steels containing sufiicient carbon to respond to a quench 9.2fm; P 237 2 g'ig' and temper type of heat treatment. As a result there are Phosphorous Zoo/i2 .06/.20. no commercial steels containing significant amounts of 33 3? Q8 5 6- phosphorous to improve the hardenability and corrosion Niekel. ioo'max. i.00 max resistance ot martensitically hardened steels. 40 gg i f The principal ob ect of this invention is to provide a method of heat treatment which will extend the use of Table I shows the compositions of two heats of medium phosphorous as an alloying element to steels containing carbon steel, the low phosphorous steel being illustrative sufiicient carbon to result in the formation of martensite of the prior art and the high phosphorous steel being an when quenched from above the upper critical temperature. example of a steel which is benefited by the heat treat- Another object of this invention is to provide a method ment of this invention.

TABLE I CMnP SSiNiCrVMoCu High Phosphorous Low Phosphorous of heat treating high carbon-high phosphorous steels to These two induction furnace heats were cast into hotimprove their notch toughness. topped, 6 /2 square X 150 lbs. ingots. The ingots were Another object is to provide a method of lowering imheated to 2200 F. and rolled to 3" square billets. The pact transition temperatures and to enhance the hardenbillets were reheated to 2200 F. and rolled to 1" diameability and corrosion resistance of martensitically hardter round bars. The 1 bars were rough machined to ened high carbon-high phosphorous steels. $5 oversize charpy specimens. All specimens were heated Still another object of this invention is to provide an to 1525 F., held at heat for one hour, and quenched in inexpensive corrosion resistant material having suflicient water. Specimens from each heat were tempered at tensile and impact strength to permit its use as a high 1000 F., 1100 F., 1200 F. and 1300 F. and quenched strength material. in water. The specimens were then finish machined to These and other objects are accomplished in the pres- V-notch charpy impact specimens and broken at various ent invention by means of a method of heat treating temperatures to obtain the 15 ft./lb. transition temperacarbon-phosphorous steels utilizing conventional martentures shown 1n Table II.

TABLE II [15 Ft. Lb. V-Notch Charpy Transition Temperatures] Temper High Phosphorous, F. Low Phosphorous, F.

1,000 F. water quench. -270 1,100 F. water quench +15 -215 1,200" F. water quench" 75 260 1,300 F. water quench -240 These results show that water quenching from a tempering temperature of 1200 F. or higher has a pro nounced efiect in improving the impact strength transition temperatures of martensitically hardened high phos- 4 having the compositions shown in Table IV and heat treated in the manner shown in Table V were subsequently tested. The results of these tests are shown in Table VI below:

TABLE VI.PHYSICAL PROPERTIES Yield Tensile Reduction 15 Ft. Lb. V-Notch Composition Treatment Strength, Strength, Elong. of Area Charpy Transition p.s.i. p.s.i. Temperature, F.

ASTM Specification 11325-64 1 77,000 1 105,000 1 14. 1 35.0 A 1 94, 700 120, 000 22. 8 61. 0 10 2 92,700 119, 000 23.0 59. 4 35 3 86, 400 113, 800 24. 8 62. 8 125 1 91, 500 116, 300 23. 5 64. 0 40 2 95, 100 121,200 21. 8 56. 8 -55 3 84, 800 112, 400 23. 5 57. 2 70 1 100, 500 124, 900 20. 3 50. 6 -15 2 99, 200 127, 500 20. 0 51. 0 35 3 93, 200 120, 600 20. 5 47. 3 -100 l 109, 800 133, 000 20. 0 52. 4 3 102,500 126, 800 21. 3 52. 7 35 4 108, 100 127, 100 19. 5 51. 6 5 99,200 120, 700 21. 0 56. 6 55 6 88, 700 117, 200 22. 5 54. 9 -100 4 110, 700 132, 400 21. 0 5G. 4 +5 5 103, 500 124, 200 16. 0 37. 9 35 6 88, 900 122, 500 19. 0 44. 1 75 phorous steels but that the same treatment had no significant etfect on low phosphorous steels.

The eifect of various cooling rates from a 1200 F. temper on the impact strength transition temperatures of martensitically hardened high phosphorus and low phosphorus steels is shown in Table III:

These results show that the effectiveness of accelerated cooling from the tempering temperature on the transition temperatures of martensitically hardened, high phosphorus steels is directly proportional to the cooling rate but that identical heat treatments fail to have any significant efifect on low phosphorous steels.

Examples of specific compositions of high strength, corrosion resistant steels that were heat treated in the manner of this invention are shown in Table IV below:

TABLE IV Compositions 0 M11 P S Si Ni Cr Cu Example:

Steels of the compositions set forth in Table IV were heat treated in the manner set forth in Table V below:

TABLE V.HEAT TREATMENTS Harden Temper 2- 1,275 F Water Quench 3- do 1,300 F. Water Quench. 4.-- 1,550" F. Water Quench 1,250 F. Water Quench. 5..- 1 275 F. Water Quench.

6 do 1,330 F. Water Quench.

Test specimens cut from 1%" diameter bars of steels These results show that the minimum tensile test 5 requirements for bolting materials shown in A.S.T.M. Specification A325-64 can be satisfactorily obtained with the heat treatment of the invention on steels having the compositions shown in Table IV, and, in addition, desirably low transition temperatures will be obtained. Thus, these materials are particularly adapted for use in the fabrication of structures requiring high strength, low alloy, corrosion resistant steels which are to be left exposed to the weather.

The heat treatment process of this invention is applicable broadly to martensitically hardened high phosphorus steels which may also contain various other alloying elements such as manganese, silicon, nickel, chromium, molybdenum, copper, vanadium, etc., either singly or in various combinations and amounts which are known to improve hardenability.

All references in the specification and claims to percentages are weight percentages.

I claim:

1. A method of tempering martensitically hardened steel consisting essentially of: .12% to .80% carbon, .06% to phosphorus, balance iron, to produce a high strength, corrosion resistant steel having a 15 ft. lb. V- notch Charpy impact transition temperature no higher than 5 F., said method comprising:

(a) heating said steel to a temperature between the Ac temperature and the Ac, minus 150 F., and (b) cooling the steel from said temperature at a rate not less than 1 F. per second. 2. The method of claim 1 in which said steel contains: Percent C .35-.55

balance iron.

balance iron.

4. The method of tempering martensitically hardened steel consisting essentially of .12% to .80% carbon, .06% to .20% phosphorus, balance iron, to produce a high strength, corrosion resistant steel having a ft. lb. V- notch Charpy impact transition temperature no higher than 5 F., said method comprising:

(a) heating said steel to a temperature between 1250 F. and 1300 F., and (b) cooling said steel from said temperature at a rate not less than 1 F. per second. 5. The method according to claim 4 in which said steel contains:

Percent Mn (max.) 1.50

Ni (max.) 1.00

Cr (max.) 1.00

Cu (max.) .50

6. The method according to claim 4 in which said steel contains:

Percent Mn .50-1.50 P .06-.12 S (max.) .05 Si .20-.90 Ni (max.) 1:00 Cr .401.00 Cu (max.) .50

7. A high strength corrosion resistant, tempered martensitic steel consisting essentially of:

Percent C .l2-.8O

P .06-.20 Mn .50-1.50 21 20 .33

1 Ni (max.) 1.00 Cr (max) 1.00 Cu (max.) .50

and the balance iron which has been heated to a temperature between the Ac, temperature and a temperature 150 F. below the Ac temperature, and cooled from said temperature at a rate of not less than 1 F. per second, said steel characterized by a 15 ft. lb. V-notch charpy impact transition temperature no higher than 5 F.

References Cited UNITED STATES PATENTS 2,315,156 3/1943 Larrabee -123 2,319,635 5/1943 Saylor 75-123 2,485,358 10/1949 Case 75-123 OTHER REFERENCES Allen et al.: Journal of the Iron and Steel Institute, vol. 182, April 1956, relied on pp. 375-388.

Hopkins et al.: Journal of the Iron and Steel Institute, vol. 188, March 1958, relied on pp. 218, 219 and 222-235.

Transactions of AIME, vol. 188, February 1950, relied on pp. 389-396.

Transactions of AIME, vol. 185, August 1949, relied on pp. 535-543.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R. 148-143, 36 

